Police and doctors are investigating the death of a Labour MP whose body was found at his home today.

Dr Ashok Kumar, 53, had been working as normal, with major commitments as parliamentary private secretary to Hilary Benn, the environment secretary. He was also campaigning for Corus steelworkers' jobs in his Middlesbrough South and East Cleveland constituency. His body was found after anxious staff failed to rouse him by phone and called emergency services, who broke into his home.

The prime minister, Gordon Brown, led tributes to the former research scientist's 14-year Commons career, saying: "He was a tenacious campaigner for his constituents and a warm and incredibly generous man." The former prime minister Tony Blair, a fellow north-east MP before his retirement, said: "Ashok was a dedicated MP, with a determination to serve the community he was from."

Cleveland police said Kumar's death was not being treated as suspicious but remained unexplained. The MP had a majority of 8,000 in 2005, but there will be no byelection because of the proximity of the general election.

Kumar was single and lived alone, but had talked recently to neighbours. He was born in India and moved to Britain to study at Imperial College in London, before moving to Teesside as a senior researcher for British Steel.

His Commons work included advocacy of humanism and opposition to faith schools, but he principally campaigned on behalf of Teesside's declining heavy industry. He first entered parliament for Langbaurgh, also on the Tees south bank, in 1991, when he became the fifth Asian-origin MP since the end of the second world war. He lost the seat the following year, but returned to win the redrawn constituency in New Labour's landslide in 1997.

Police said paramedics had declared him dead at the scene and that relatives had been informed. Benn said: "It is very hard to believe that Ashok is no longer with us. He was a pioneer, a doughty fighter for his constituents and a Labour man through and through, who cared deeply for others. He was also fearless in pursuit of what he saw as right."

The death was announced to the Commons by the Speaker John Bercow, who said: "Ashok was a most assiduous member, much respected by the House."

Meanwhile, the MP for one of Labour's most marginal seats announced today she would not defend her 37-vote majority at the election. Laura Moffatt's decision gives party workers in Crawley, east Sussex, just weeks to find a new candidate.

Labour's national executive committee will draw up shortlists for her and Kumar's seats, as well as Blackpool North and Fleetwood, where Joan Humble announced her retirement late last month.